Hydraulic suspension systems are similar inventions, they all act as an active body control and can adjust the height, which big companies all around the world put their own name on them such as ABC or Active Body Control. There has been 6 decades since its generation, but the evolution continues until the difficulties to be solved.
Idea of using Hydraulic systems for vehicles suspension mechanism was first discussed in Citroen Car Company in 1952. Designers of this company used all hydraulic mechanisms ever invented in manufacturing the Citroen DS219.
The principles of hydraulic suspension system, sometimes known as hydro pneumatic, are based on compressibility of gases and non-compressibility of liquids. Each used kickstand being used in suspension system replacing normal springs/coils consists of one simple cylinder, one simple piston, and one storage or warehouse which is under pressure of nitrogen gas and is being installed on top of the kickstand.
Hydraulic oil can move back and forth between kickstand and storage. The auto body weight causes piston rise in cylinder resulting oil emission from kickstand and its entrance to the storage. With the addition of oil into the store, nitrogen gas-locked inside-density increases in order to reach equilibrium with the weight of the car.
Thus the locked nitrogen gas acts like a coil/spring. By placing an orifice (narrow channel) between piston and storage we can decrease the velocity in pistons and absorb the uneven impact impulses—in conventional vehicles this action is carried out by shock absorbers.
But the system has some shortcomings and weaknesses as follow:                High maintenance costs        High production costs        Vehicle confusion in consecutive loops        Malfunctioning in bumps in high speeds        Using significant portion of motor energy on system performance        Debility in adjusting the vehicle's height as it is moving or lack of driving ability at different heights (according to manufacturers, this system can adjust vehicles height but due to the loss of driving comfort and safety cannot move other than the standard heights).        
In this system, each of the four Shock Absorbers and springs/coils are individually controlled by electronic control unit. In this case, the sensors mounted in each wheel give the momentary reports about the vehicle and state of the road to the control unit. Thus, comparing to vehicles using ABC, the vehicles that are equipped with this system have lower fuel consumption and more power engine. For a better understanding, in stable auto conditions (such as highways, straight paths) using signals from sensors the system sends commands which could increase the flexibility of air coils and cause superior comfort.
However if the sensors transmit information including imbalance vehicle or entering a hairpin, the control unit takes control of each four wheel, the coils/springs, and shock absorbers in order to pass the encountered crisis. Another advantage of this system is that when the driver is forced to make a severe and sudden brake the system prevents the front parts of the car to move downwards and keeps the car balanced.
Scientific basis of this system is the centrifugal force. Due to the effect of this force, cars tend to reduce height in every curve and falloff the path. If we could stop the height reduction then we can prevent misbalancing and falling-off's as well.
In this system each wheel and coil are being checked individually and are equipped with a smart barometer.
In other words, although it is a great comfort it also provides safety to the vehicle. On the other hand this system is capable of driving in any desirable height without loss of comfort and safety.